This invention relates to electric field sensors in the medical field for the detection of alternating electrical fields originating from within the body to produce electro-cardiograms (ECGs) and electro-encephalograms (EEGs) and the like, as well as heart rate monitoring. It also relates to other applications for sensing external electric fields.
The detection of electrical potentials occurring on the human body is the basis for ECG/EEG diagnostic procedures used to assess heart conditions and brain functions (hereafter xe2x80x9cECGxe2x80x9d). An extensive science has been established on the basis of coupling conductive electrodes to the human body to sense the low-level electrical signals that the body is able to generate.
A feature of this technology in the past has been to focus on reducing electrical resistance at the skin/electrode interface. For this purpose ECG electrodes are often used in conjunction with conductive gels and suction cup attachment mechanisms. These arrangements are uncomfortable for the user, restrict mobility, and have limited useful life.
Dry Electrodesxe2x80x94Prior Art Approach
Investigations have been made into using capacitive pickups to detect electrostatic potentials on the skin of a patient. Examples in the literature include the text xe2x80x9cIntroduction to Bio-Electrodesxe2x80x9d by Clifford D. Ferris, published by Plenum Press in 1974. In this text the author discusses experiments with insulated, capacitive electrodes based upon the configuration (page 184):
xe2x80x9cBody surface (skin)/Dielectric/metal/FETxe2x80x9d.
A shielded single electrode and a two-electrode circuit based on a capacitive electrode are depicted on page 185. Electrode capacitance is reported as 14 uF/cm2 at page 187.
The text xe2x80x9cElectrodes and Measurement of Bio-Electric Eventsxe2x80x9d by L. A. Geddes, published in 1972 by Wiley-Interscience discusses xe2x80x9cdry electrodesxe2x80x9d at pages 98-103. A single electrode circuit based on a insulated anodized electrode and FET transistor is depicted at page 100. A value for electrode capacitance is reported at page 102 as being 3200 picoFarads Capacitance ranges of 5000-20000 picoFarads/cm2 are referenced at page 102. In particular, this reference reports (page 102):
xe2x80x9cAt present there are attempts to provide ultra thin films of insulating materials having high dielectric constants and strengths so that a high electrode-to-subject capacitance will be attained . . . xe2x80x9d.
This statement recites that obtaining a high level of capacitive coupling is an objective and necessarily presumes that such electrodes will be placed in intimate contact with the body of the subject being measured.
In the text xe2x80x9cPrinciples of Applied Biomedical Instrumentationxe2x80x9d 2nd edition, L. A. Geddes, L. E. Bater published by Wiley Interscience, 1975, the author observes (at page 217):
xe2x80x9cTo obtain an electrode-subject impedance that is as low as possible, every effort is made to obtain a high capacitance by using a very thin dielectric having a high dielectric constant.xe2x80x9d
Capacitance values from 5,000 pF/cm2 to 20,000 pF/cm2 are cited.
A Technical Note entitled xe2x80x9cNew Technologies for In-Flight, Pasteless Bioelectrodesxe2x80x9d by D. Prutchiand A. M. Sagi-Dolev, published in Aviation, Space and Environmental Medicine, June 1993 (page 552) describes a capacitive, dry bioelectrode for obtaining EEG and ECG signals obtained through a plate anodized with aluminum oxide. Coating thicknesses of 50 um and 170 um are referenced. Allowing for a dielectric value of 10 (for aluminum oxide) this thickness would provide an electrode with the ability to develop a capacitance of about 50 pF/cm2 to 180 pF/cm2, if intimately presented to a conducting surface.
Accordingly, the prior art has addressed the problem of capacitive dry electrodes in terms of developing high capacitive values for insulated electrodes placed in intimate contact with the surface being monitored. These prior investigative efforts have been focused on maximizing the coupling between the electrode and the skin surface carrying the potential to be detected. This has led to electrodes that employ thin dielectric surfaces that are capable of providing capacitive values from about 50-1000 picoFarads/cm2 and higher. It is a necessary adjunct to establishing high capacitive coupling to a body that the electrodes be pressed intimately against the surface being sensed, and that the surface be smooth and free of defects.
True Effective Capacitance
It is believed that all of the capacitive values cited in the prior art references are based on the premise that cited capacitance values are for the maximum capacitance that an insulated electrode can develop when pressed against a conductive surface.
A capacitive pickup electrode for an ECG system may be designed to have a capacitive value of several hundred picoFarads per square centimeters when its insulated plate surface is laid over a smooth, highly conductive counter-electrode surface, such as a sheet of copper. This is the condition for maximum capacitance. However, when placed proximate to the human skin, the dead layer of the skin acts effectively as an insulating spacer, removing the plate of the pickup electrode further from the source of the electric field being sensed. In such a configuration, the effective value of the capacitive coupling between a typical, high capacitance pickup electrode e.g. 100+ pF/cm2 and the field source within the human body may be on the order of 1-100 picoFarads/cm2 depending on the intimacy of contact with the body and the presence of sweat or hair on the skin. The prior art has endeavoured to maximize this capacitance value.
Difficulties of Intimate Coupling
The results of prior art endeavours have been only moderately successful. One problem that has arisen is the extensive sensitivity of these capacitive electrodes of prior design to variations in the gap or intimacy of contact between the electrode and the skin. When intimate contact is the objective, even the presence of hair or sweat can cause variations in the value of capacitive coupling being established. The procedure of pressing dry electrodes against the body has presented similar inconveniences to those arising in the use of conductive electrodes, e.g., discomfort and limited mobility due to intimate contact protocols. In particular, prior art systems have never been reported as operating through clothing fabric. No proposal has been made to obtain alternating electrical signals of the ECG, EEG type, etc. through use of dry capacitive electrodes that are not positioned at fixed locations on the skin surface of a subject.
Further difficulties associated with the use of dry electrodes pressed into intimate contact with the skin of a person are tribo-electric effectsxe2x80x94electrical charges created by sliding friction and pressure. Tribo-electric effects deliver large, essentially static charges, to the pickup electrode.
Such charges impose a near DC or very low frequency drift in the background level over which the more relevant, higher frequency signals are imposed. To discharge the amplifier input and pickup electrode of such capacitively acquired charge, the input resistive impedance of the high impedance first stage amplifier should be carefully selected.
Thus a particular concern when sensing alternating signals is the band-pass capabilities of the sensing system. Ideally, the pickup electrode should drive an amplifier with a complementary input impedance which, in the case of ECGs is able to process low level, e.g. milli-volt, signals in the range 0.05 Hz to 150 Hz. The lower cut-off frequency should be stable in order to restore the bias value of the driven amplifiers to its normal value in cases where the circuit is over-driven by a very low frequency or DC offset signal.
To minimize the disruptions caused by very low frequency or DC over-driven off-sets, the capacitive coupling to the body (C) should be matched to the input impedance of the amplifier sensor (R) via a preferred, tuned RC-relation. This allows the sensor to have a stable band pass. U.S. Pat. No. 3,744,482 addresses this issue with a tuned feed-back loop. However, for the tuning of the sensor input to be consistent, both the resistive xe2x88x92R and capacitive xe2x88x92C values should be stable.
Variance in Capacitance
A pickup electrode may be of such a design as to permit it to achieve high value capacitive coupling, as for example maximum values of 50-100+ picoFarads/cm2 when placed on a conductive plate. This can be effected through use of thin or high dielectric value insulative layers. A difficulty arises, however, in ensuring that the frequency cut-off of the RC network at the input stage is appropriately tuned when the pickup electrode is capable of high capacitance coupling. This difficulty arises from the fact that a pickup electrode with potentially high capacitance will exhibit varying actual capacitive coupling values when placed adjacent to the body generating the electric field, particularly when an attempt is made to place such an electrode in intimate contact with the skin of the human body being sensed.
By example, the actual capacitive coupling value may range over several hundred percent if the electrode is pressed very tightly against skin wetted with body sweat. In this situation, since capacitance varies inversely with the gap separating with the capacitor electrodes, the system is operating in the separation-sensitive region of a graphic plot of capacitance vs Separation Distance (of FIG. 5).
When the effective capacitance of the pickup electrode varies substantially, the cut-off value of the RC filter arrangement will vary correspondingly. This will reduce the performance of the RC combination as a well-tuned, high-pass, low frequency cut-off filter. Settling times for low frequency signal artifacts will be lengthened as the capacitive value of C is doubled or tripled.
Background Noise Rejection
A major source of noise for electronic systems is ambient 60 Hz signals (in North America) arising from the power system. It is known that sixty hertz background noise can be eliminated or greatly reduced through the use of a differential amplifier arrangement. However, for maximum rejection of common mode noise to be achieved, the inputs to both branches of the differential amplifier should be fully balanced. If the inputs are not balanced improper signal differencing will occur and the output will be disturbed by the imbalance. In the case of ECG systems, balance would ideally be achieved by having two separate ECG pickup electrodes couple to the source body originating the electrical field with the same degree of capacitive coupling.
Where intimate-contact, high capacitance electrodes are employed, this balancing is hard to maintain. A need exists for a more stable system to be employed for these types of applications. The invention herein addresses this need.
In summary, a need exists in the medical field to provide an electrical field sensor for detecting alternating signals that is less demanding in terms of electrode/body coupling. In non-medical fields, useful applications may also arise where the measurement of an oscillating surface charge is to be effected without contact arising between the charged surface and the electrical sensor. The invention herein addresses such needs.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.
According to one aspect of the invention the signal pickup procedure for obtaining an electric field or ECG signal and the like is carried-out under a configuration wherein the effective capacitance coupling the electrical field source to a high impedance sensing amplifier is relatively insensitive to variations in the separation between the body that serves as a field source and the pickup electrode. Small displacements of the pick-up electrode lead to little change in the degree of capacitance coupling between the electrical field source and the sensing amplifier.
According to the invention in one aspect, an electric field sensor is provided that includes a first pickup electrode for placement next to a surface whose electrical field is to be sensed through capacitive coupling. This pickup electrode is not operated, as in the past, to achieve high capacitive coupling values for such electrodes, i.e. operating in the separation-sensitive region of a Capacitance vs. Separation Distance graph (as per FIG. 5). Rather, by the arrangements of the present invention, the value of the capacitive couplings between the source field and the sensing amplifier is kept small i.e. under 40 picoFarads/cm2, preferably 20 picoFarads/cm2, more preferably, 1-10 picoFarads/cm2. This may be achieved by avoiding intimate contact with the body e.g. by positioning the plate of the pickup electrode at a xe2x80x9cstand-offxe2x80x9d location that reduces the sensitivity of the measured output to motion effects i.e. variations in the separation of the pick-up electrode from the surface of the body being sensed. And it may be achieved by placing a limiting capacitor in series with the input to the sensing amplifier.
To ensure the xe2x80x9cstand-offxe2x80x9d effect of the invention first arrangement is achieved, an insulating layer may be provided over the electrode to separate it from a body by a gap that ensures that capacitive coupling does not vary sensitivity with separation. In some cases, useful signals can be obtained by placing sensors of the invention over protective layers already present on the body.
The objective in designing the sensor in accordance with this criterion is to ensure that the overall, effective capacitance formed between the pick-up electrode and any surface that may be presented to the outer face of the pick-up electrode will always have a value in the region of a plot of capacitance value versus separation distance wherein, upon displacement of the electrode by a standard amount, the capacitance is varied by a limited percentage value.
Equivalently, changes in the surface condition of the field-emitting object, e.g. the appearance of sweat on skin, does not significantly change the degree of capacitive coupling that is present when the sensor is operating under the conditions of the invention.
In particular, and preferably, when the separation of the electrode from the surface varies by 0.1 mm or less, the capacitance value of the coupling between the body and the pick-up electrode varies by less than 50%. More preferably the capacitive value varies by less than 20%.
By providing an ECG pickup with an insulative layer that precludes the pickup electrode from achieving capacitance values of higher than a specific value, e.g. 40, 20 or less, preferably 10 picoFarads/cm2, an ECG system so equipped will be inherently suited for operation in the preferred, second, separation-insensitive region (as per FIG. 5) . The presence of such a capacitance-limiting insulative layer will preclude an electrode from operating in the first, separation sensitive zone.
It is preferable for the insulating layer to have a thickness which is equal to, or greater than, the size of surface irregularities of the body being measured, and equal to or greater than the variations in the sensor-to-body separation gap.
This is completely counter-intuitive to the methodologies applied by the prior art experiments with capacitive, xe2x80x9cdryxe2x80x9d electrodes which employ extremely thin dielectric layers and then proceed to place the sensor in intimate contact with the surface of the body being sensed.
Thus, the present invention, in one aspect, employs a dielectric layer for the pick-up electrode that ensures that sensing is occurring at a stand-off location which is insensitive to minor motion and/or surface irregularities as well as temporal changes in surface characteeristics.
The instability arising from the variations in the coupling capacitance of the pickup electrode can be addressed in a further manner, namely by inserting into the input of the high impedance sensing amplifier that receives signals from the pickup electrode a series capacitor of fixed and limited value. This limiting capacitor should preferably have a minimum value that is greater than the input capacitance of the amplifier stage that is driven by the signal received from the body through both the pickup electrode and the limiting capacitor. As a preferred upper limit, the limiting capacitor may have a value that is less than the effective coupling capacitance between the pickup electrode and the body. Values for this limiting capacitor outside this preferred range may also be adopted. The inclusion of such a series capacitor has the same effect in constraining variations in the effective, overall capacitance value of the coupling between the electrical field source and the input amplifier as the xe2x80x9cstan-doffxe2x80x9d variant of the invention referenced above.
When this alternate procedure for rendering the input amplifier relatively insensitive to the electrode/body separation distance, e.g. placing a limiting capacitor in series at the input to the first stage amplifier, is employed, use of a series limiting capacitor of appropriate value, e.g. 40 picoFarads, will set an upper limit on the capacitance coupling between the field source and the input amplifier. As the pickup electrode is in series with the limiting capacitor, the combined capacitance of the two cannot exceed the value of the limiting capacitance. Because the value of the limiting capacitor is fixed, the RC value for the high pass filter at the input stage is stabilized. Even if the pickup electrode has a relatively high maximum possible capacitance, e.g. over 1000 picoFarads, because it is in series with the limiting capacitor, it cannot absorb a substantial static charge. Viewed alternately, if the pickup electrode were to achieve in fact, a very high level of capacitance coupling to the body, at a value greatly exceeding the capacitance value of the limiting capacitor e.g. 10:1, then we may treat it as having a minimal, or transparent impedance contribution to the combined series capacitance of the amplifier""s input. This will still leave the limiting capacitor, e.g. 40 pf as dominating the capacitive coupling between the field source and the input amplifier.
Voltage Divider Network
In detecting electric field signals through the capacitive pickup arrangement of the invention, the signal being sensed by the input amplifier is essentially being taken from across a voltage divider network defined by the pickup electrode, the limiting capacitor (if present), the input capacitance of the amplifier and the remaining electrical coupling (either resistive or capacitive or both) at the other end of the voltage divider network which is connected to the body which is the source of the electric field. Assuming this last connection is of relatively low impedance, the signal strength seen at the input to the amplifier depends on the ratio of the input capacitance of the amplifier to the other capacitors in the series chain. If the input capacitance of the amplifier is small, then most of the signal strength will appear across this capacitance, and be sensed by the amplifier.
In actual use, the effective capacitive value of the pickup electrode may be on the order of the value of the limiting capacitor. In this case, its impedance contribution will become significant. For example, the pickup electrode effective coupling capacitance being equal in value to that of the limiting capacitancexe2x80x94e.g. 40 picoFaradsxe2x80x94then the combined, net capacitance of these two elements in series would drop to half of their individual capacitance values e.g. 20 picoFarads. This will not, however, have a serious deleterious effect on the signal detection performance of the overall system so long as the input capacitance to the high impedance amplifier is small e.g. 2-5 picoFarads.
Differential Amplifier/Dual Inputs
As is done in the case of conductive electrode ECG systems, two pick-up sensors may be applied at two distinct locations on the skin. By taking the difference in the output signals from two locations on the body the benefits of common mode noise rejection may be obtained. The objective of minimizing variations in such capacitance values is also important for this special case arrangement in ECG-measuring systems: the use of dual input differential amplifiers to obtain rejection of common mode noise.
Differential amplifiers used to reject common mode noise, e.g. ambient 60 Hz, fail to achieve full rejection when the bias levels of the amplifiers are imbalanced or if the amplifiers have unequal RC characteristics. To maximize the prospects that these levels and characteristics are balanced, both branches of the pickup elements should have similar settling times when disrupted by an off-setting, very low frequency signal. This requires that the effective capacitance of the couplings within both branches between the sensed body and amplifier inputs be similar. The invention addresses means for achieving this last criterion.
Clothing-Supported Arrays
On the foregoing basis, this invention provides a means for detecting electrical fields present on the surface of a body without the use of conductive gels and suction-based appliances. Useful signals may be obtained based on the combination of multiple electrodes assembled in a fixed, preformated array. Thus, multiple electrodes, e.g. 4 or more, may be carried by a clothing-type of support as an array that can be readily donned or removed with minimal inconvenience. This provides considerable freedom for the tele-monitoring of patients while they engage in daily routines. Freedom from the limitations of conventional tele-monitoring arrangements represents a valuable advance in this field.
The foregoing summarizes the principal features of the invention and some of its optional aspects. The invention may be further understood by the description of the preferred embodiments, in conjunction with the drawings, which now follow.